This application claims the benefit of Korean Patent Application No. 2001-80902, filed Dec. 18, 2001, in the Korean Industrial Property office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an ink-jet print head, and more particularly, to an inkjet print head having a nozzle plate, a heat element formed on the nozzle plate, and a thermal shunt formed in the nozzle plate such that thermal accumulation on the nozzle plate can be effectively prevented
2. Description of the Related Art
Ink ejection mechanisms of ink-jet print heads include an electro-thermal transducer having a heat source generating bubbles to eject ink by using a bubble-jet method, and an electromechanical transducer having a piezoelectric device varying a volume of the ink caused by deformation of the piezoelectric device to eject the ink.
The bubble-jet method of the electro-thermal transducer is classified into a top-shooting method, a side-shooting method, and a back-shooting method according to a relationship between a growing direction of the bubbles and an ejecting direction of an ink droplet of the ink. In the top-shooting method, the growing direction of the bubbles is the same as the ejecting direction of the ink droplet, in the side-shooting method, the growing direction of the bubbles is perpendicular to the ejecting direction of the ink droplet, and in the back-shooting method, the growing direction of the bubbles is opposite to the ejecting direction of the ink droplet.
A basic principle of the back-shooting method and a structure of an ink-jet print head using the same are disclosed in U.S. Pat. No. 5,760,804 to Heinzl et al. issued Jun. 2, 1998. In addition, various structures used for the back-shooting method are disclosed in U.S. Pat. No. 4,847,630 to Bhaskar et al. issued Jul. 11, 1989 and U.S. Pat. No. 6,019,457 to Silberbrook issued Feb. 1, 2000.
FIG. 1 is a cross-sectional view of a conventional ink-jet print head.
A chamber 1a having a hemispheric shape is formed in a substrate 1, which is formed of silicon, etc., and an ink inlet 1b connected to an ink supply source (not shown) is formed in a lower portion of the chamber 1a. A nozzle plate 2 is formed on the substrate 1 and above the chamber 1a, a nozzle 3 is formed in the nozzle plate 2, and an ink droplet 15a is ejected from the nozzle 3.
The nozzle plate 2 includes a thermal insulation layer 2a and a chemical vapor deposition (CVD) overcoat 2b formed on the thermal insulation layer 2a. The insulation layer 2a and the CVD overcoat 2b correspond to a portion of the substrate 1. The insulation layer 2a has a first surface facing the substrate 1 and a second surface contacting the heat element 8.
A heat element 8 is disposed adjacent to the nozzle 3 to surround the nozzle 3. The heat element 8 is disposed in an interface area between the thermal insulation layer 2a and the overcoat 2b, and a thermal shunt 9 transferring heat from the heat element 8 to ink 15 in the chamber 1a and transferring redundant heat to the substrate 1 through the insulation layer 2a is formed above an upper side of the heat element 8.
In the conventional ink-jet print head, if a current pulse is applied to the heat element 8, the heat is generated from the heat element 8, and bubbles 7 are formed from the first surface of the insulation layer 2a. After that, while heat is continuously generated from the heat element 8, the heat is continuously supplied to the bubbles 7, and thus the bubbles 7 expand. Due to the expansion of the bubbles 7, pressure is applied to the ink 15 disposed in the chamber 1a, and thus the droplet 15a of the ink 15 in a vicinity of the nozzle 3 is ejected to an outside of the nozzle plate 2 through the nozzle 3. After that, additional ink 15 is sucked into the chamber 1a along an ink channel or passage direction 5, and thus the chamber 1a is refilled with the additional ink 15.
In the conventional ink-jet print head using the back-shooting method, as described above, the heat element 8 arranged around the nozzle 3 of the nozzle plate 2 is formed between the insulation layer 2a and the overcoat 2b, which constitute the nozzle plate 2, and the heat element 8 is connected to an electric line (not shown) to receive current from a power source. The electric line is also formed between the insulation layer 2a and the overcoat 2b. 
If the current is supplied to the heat element 8, heat generated from the heat element 8 is transferred to the ink 15 in the chamber 1a, and thus the bubbles 7 are formed in the ink 15. However, remaining redundant heat may be accumulated on the nozzle plate 2, but the thermal accumulation of the remaining redundant heat is prevented by the thermal shunt 9. In other words, the thermal shunt 9 prevents the thermal accumulation on the nozzle plate 2. The temperature of the nozzle plate 2 raised by the remaining redundant heat, which is has not been transferred to the ink 15 in the chamber 1a, is lowered when the remaining redundant heat is transmitted to the substrate 1. If the temperature of the nozzle plate 2 is increased to more than a predetermined temperature, a lifetime of the ink-jet print head is shortened, and the performance of an ink-jet ejection operation is lowered. The problem with the thermal accumulation may not occur in a structure in which the heat element 8 is directly formed on the substrate 1 but occurs in another structure having the heat element 8 formed on a portion spaced-apart from the substrate 1, for example, on the nozzle plate 2 having a membrane structure with a large heat transfer resistance as shown in FIG. 1.
Likewise, in the ink-jet print head having the heat element 8 formed on the nozzle plate 2, the thermal shunt 9 is used to improve the above thermal accumulation. However, with the thermal shunt 9 of the conventional ink-jet print head, it is very difficult to efficiently transfer or radiate the remaining redundant heat to the substrate 1. In addition, the thermal shunt 9 is made of a conductor, such as aluminum, and is extended above the heat element 8 and between upper and lower material layers. Since the thermal shunt 9 is disposed very close to the heat element 8, cracks are generated due to the thermal stress caused by a difference between thermal expansion coefficients of the thermal shunt 9 and the upper and lower material layers.
To solve the above problems, it is an object of the present invention to provide an inkjet print head, which is capable of more effectively preventing excessive thermal accumulation on a nozzle plate.
Additional objects and advantageous of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Accordingly, to achieve the above and other objects, there is provided an ink-jet print head. The ink-jet print head includes a substrate, a channel formed on the substrate to supply ink in an ink passage direction, a nozzle plate connected to the substrate and including a nozzle corresponding to the channel, a heat element disposed in the nozzle plate to surround the nozzle, a thermal conduction layer formed on an upper side of the heat element, an intermediate insulation layer formed between the thermal conduction layer and the heat element, and a first thermal shunt spaced-apart from the heat element by a predetermined interval in a direction parallel to a major surface of the nozzle plate not to overlap the heat element and connecting the thermal conduction layer to the substrate.
The thermal conduction layer is made of diamond like carbon (DLC) or silicon carbide (SiC), and a passivation layer is formed on an upper surface of the thermal conduction layer, and a hydrophobic layer is formed on the passivation layer.
An electrode applying current to the heat element is formed on the nozzle plate, and the first thermal shunt is formed of the same material as that of the electrode.
The first thermal shunt includes first and second metal layers formed on the nozzle plate, an insulation layer is formed between the first and second metal layers, and a first through hole formed on the insulation layer to allow the first and second metal layers to contact each other. Here, the first through hole is spaced-apart from a wall defining the chamber so as not to thermally affect the ink in the chamber. The electrode includes a first electrode directly connected to the heat element and a second electrode formed on an upper layer formed on the first electrode, an insulation layer formed between the first electrode and the second electrode, and a second through hole formed on the insulation layer to allow the first electrode to be electrically connected to the second electrode. Thereby, a second thermal shunt including the first and second electrodes is provided. The first and second thermal shunts surround the heat element at a predetermined interval.
The above and other objects are achieved by providing a structure in which redundant heat generated from the heat element can be effectively transferred to a bulk silicon substrate in the ink-jet print head using a back-shooting method in which the heat element is spaced-apart from the substrate. That is, the inkjet print head includes a membrane. The chamber having a hemispheric shape is formed in the membrane, and the nozzle is formed above the chamber of the membrane. A thermal conduction layer is made of the DLC or the SiC to absorb the heat generated from the heat element and formed above the heat element with by the predetermined interval in the direction parallel to the major surface of the nozzle plate or parallel to a plane disposed between the nozzle plate and the substrate. A thermal shunt or bridge is formed between the thermal conduction layer and the substrate and spaced-apart from the heat element to rapidly transfer the heat from the thermal conduction layer to the substrate. An insulation layer having a predetermined thickness is made of a material having thermal conductivity lower than the DLC, such as an inter-metal dielectric (IMD) material, and disposed between the thermal conduction layer and the heat element, and thereby preventing the heat generated from the heat element from being excessively absorbed into the thermal conduction layer. Due to the excessive absorption and exhaustion of the heat, it is very difficult to effectively generate the bubbles.
The thermal conduction layer has an electrical insulation characteristic and is made of an inorganic material having a very high thermal conductivity and a low thermal expansion rate lower than a metal. As a result, the occurrence of the cracks caused by the thermal stress is prevented. The thermal shunt connecting the thermal conduction layer to the substrate is spaced-apart from the heat element by the predetermined second vertical distance and is simultaneously formed with the electrode constituting an electric circuit for the heat element. Thus, a design for the thermal shunt is applied to a mask forming the electrode in the nozzle plate when the electrode is formed, and thereby the thermal shunt is formed together when the electrode having one or two metal layers is formed.